> static pte_t kvmppc_lookup_pte(pgd_t *pgdir, unsigned long hva, bool writing,
> - unsigned long *pte_sizep)
> + unsigned long *pte_sizep, bool do_get_page)
> {
> pte_t *ptep;
> unsigned int shift = 0;
> @@ -135,6 +136,14 @@ static pte_t kvmppc_lookup_pte(pgd_t *pgdir, unsigned long hva, bool writing,
> if (!pte_present(*ptep))
> return __pte(0);
>
> + /*
> + * Put huge pages handling to the virtual mode.
> + * The only exception is for TCE list pages which we
> + * do need to call get_page() for.
> + */
> + if ((*pte_sizep > PAGE_SIZE) && do_get_page)
> + return __pte(0);
> +
> /* wait until _PAGE_BUSY is clear then set it atomically */
> __asm__ __volatile__ (
> "1: ldarx %0,0,%3\n"
> @@ -148,6 +157,18 @@ static pte_t kvmppc_lookup_pte(pgd_t *pgdir, unsigned long hva, bool writing,
> : "cc");
>
> ret = pte;
> + if (do_get_page && pte_present(pte) && (!writing || pte_write(pte))) {
> + struct page *pg = NULL;
> + pg = realmode_pfn_to_page(pte_pfn(pte));
> + if (realmode_get_page(pg)) {
> + ret = __pte(0);
> + } else {
> + pte = pte_mkyoung(pte);
> + if (writing)
> + pte = pte_mkdirty(pte);
> + }
> + }
> + *ptep = pte; /* clears _PAGE_BUSY */
>
> return ret;
> }
So now you are adding the clearing of _PAGE_BUSY that was missing for
your first patch, except that this is not enough since that means that
in the "emulated" case (ie, !do_get_page) you will in essence return
and then use a PTE that is not locked without any synchronization to
ensure that the underlying page doesn't go away... then you'll
dereference that page.
So either make everything use speculative get_page, or make the emulated
case use the MMU notifier to drop the operation in case of collision.
The former looks easier.
Also, any specific reason why you do:
- Lock the PTE
- get_page()
- Unlock the PTE
Instead of
- Read the PTE
- get_page_unless_zero
- re-check PTE
Like get_user_pages_fast() does ?
The former will be two atomic ops, the latter only one (faster), but
maybe you have a good reason why that can't work...
Cheers,
Ben.
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